Inertial navigation systems (INSs) supply navigation data for air, space, and water vehicles. One such INS that supplies navigation data for water vehicles is the AN/WSN-7.
A typical INS includes a computer and a gimbaled inertial measurement unit (IMU) for determining the position and velocity, attitude (roll, pitch and heading), and attitude rates of the vehicle. The gimbaled IMU may include a plurality of gyroscopes to measure vehicular angular rates about each of three axes. After receiving initial attitude data from an external source, the computer of the INS determines the attitude of the vehicle using attitude-change data received from the IMU.
Indexing is the periodic reorientation of IMU sensor axes in order to average out random walk errors in the accelerometers. In this process, the IMU is periodically rotated with roll and azimuth gimbals to another orientation. The order of gimbal motions repeats every certain number of orientations. The gimbal motion occurs over a certain time period.
Readout error in the orientation of the gimbals is a bias in the attitude determined by the gyroscopes. There are methods for estimating the biases in the roll, pitch and heading of INSs due to indexing. One known method is the Gyro Indexing Compensation Algorithm (GICA) that estimates the biases in the roll, pitch and heading of the earlier mentioned AN/WSN-7.
Accordingly, a method and system for estimating the onset of regular (up to clock drift) IMU gimbal motion is needed for use in bias estimation of the roll, pitch and heading outputs.